Can I import the demo content on an existing installation?

It would work on an existing install, however you need to install the WordPress Reset Plugin and reset (delete the content) the database to guarantee the best performance of the theme.

* Please be aware that once you use this plugin, all your current database will be deleted.

WP Reset Plugin – This plugin is the fastest way to get rid of content. However, it will remove all content from your database and leave the default theme activated. It does not remove plugins, but will deactivate them and remove all pages, posts, menus, sliders, widget data, theme options, etc. Basically it removes all the content from the existing site. Click Here To Download.